In order to control the mode quality of the output beam from a laser resonator, it is a common technique to take laser rods having a uniform concentration as received from the supplier and use additional optical components within the laser resonator. Such additional components may include hard or apodizing apertures. The disadvantage of such an approach is the necessity of using these additional optical components, which adds complexity, cost, and additional optical surfaces susceptible to optical damage.
A recent patent by Robert W. Bryen and David S. Sumida entitled “Solid State Devices with Radial Dopant Valence Profile” (U.S. Pat. No. 6,996,137, issued Feb. 7, 2006) suggests that the concentration of active laser ions in the near surface region may be decreased by treating the laser rods in a reducing environment at high temperature. The associated mechanism with this treatment is the removal of oxygen ions from the structure that results in a valence reduction of active laser ions in the same region. The valence reduction changes the active ions to an inactive state with respect to the desired optical absorption required for lasing. While this approach has proven to be valid, the process relies on the diffusion of oxygen vacancies into the host structure to create a layer of decreased active ion concentration. Due to the slow diffusion rate of the vacancies, the process affects only a very small near surface region of the laser rod. Calculations show that at temperatures near the melting point of the host, a time period of greater than 10 days is required to affect 50% of the active ions in a layer less than 0.1 mm in thickness from the surface. This process is therefore too slow to be effective for the desired change in the laser rod.
The facile process of using an oxidation step to convert Yb2+ to Yb3+ ions in a YAG host has been previously been disclosed by M. Kreye and K. Becker, “An optical in-situ study of the re-oxidation kinetics of mixed valent Yb3Al5O12”, Phys. Chem. Chem. Phys. 5, 2283 (2003). However, Kreye et al report only the process parameters necessary to create the oxidation. Hence, their process produces the opposite concentration profile and not the laser active concentration profile desired as taught in the present invention.